Production of isoprene



United States Patent Office 3,206,520 Patented Sept. 14, 1965 3,206,520 PRUDUCTHON F ISOPRENE Byron W. Turnquest, Chicago, and Emmett H. Bur-h, .lr., Hazel Crest, lll., assignors to inclair Research, line, Wilmington, Del., a corporation of Delaware No Drawing. Filed Dec. 5, 1961, Ser. No. 157,269 4, Claims. (Cl. 266-680) isoprene is increasing. The availability and cost of isoprene, however, present formidable barriers to the commercial production of synthetic natural rubber. Thus, there is a continuing search for new sources of isoprene production.

A great deal of research has been directed recently to the production of isoprene by dehydrogenation of C stocks by processes similar to those employed for the production of butadienes. However, these processes have disadvantages which limit their usefulness. Dehydrogenation of isopentane, which is available in relatively large quantities from normal refinery streams and natural gas or field gas streams, is ordinarily carried out by a one step process such as a process using a chromium-aluminum catalyst. In this process, isomerization occurs during dehydrogenation resulting in the formation of one pound of piperlylene (straight chain C diolefins) for every 2.5 pounds of isoprene in the product.

Furthermore, the quantity of isoarnylenes which are available from catalytic cracking or steam cracking of gas oil is quite limited relative to the potential demand for isoprene according to the predicted potential demand for isoprene according to the predicted shortage of 600,000 long tons of natural rubber by 1965. Moreover, none of the special catalytic methods is capable of effecting dehydrogenation to isoprene in a desirably selective manner. In all cases, an appreciable amount of the hydrocarbon feed and/or reaction products is consumed in side reactions such as cracking, polymerization and oxidation.

It has also been reported that the thermal cracking of olefins such as 2-methylpentene-2 affords a relatively simple method for isoprene production. In obtaining maximum selective yields under this process, however, it has been found that the rate of cracking is relatively low. Low conversion levels are disadvantageous in that they complicate the separation of the isoprene from the crude pyrolyzate stream.

Emmett H. Burk, Jr. and William D. Hoffman disclose in their application Serial No. 122,082, filed July 6, 1961, which is a continuation-in-part of application Serial No. 94,955, filed January 11, 1961, now abandoned, a process for the production of high selective yields of isoprene by the thermal cracking or pyrolysis of 3,3dimethyl-a-olefins and that the process can be run at high conversion levels (e.g.::or about 75%) without formation of n-pentanes. Also disclosed in the above application is that the thermal cracking of 3,3-dimethyl aolefins in the presence of hydrogen halide as a catalyst results in high selective yields of isoprene at high conversion levels with large concentrations of isoprene in the C reaction product.

We have now found an improved yield of isoprene and a higher concentration of isoprene in the C fraction resulting from the pyrolysis of a feed selected from 3,3-

dimethyln-olefins, 1-halo-3,3-dimethyl alkanes or mixtures thereof, in the presence of hydrogen halide can be obtained by conducting the cracking reaction in the presence of added or extraneous C -C tertiary olefins. The importance of increasing the isoprene concentration in the C fraction, however slight, cannot be over emphasized particularly from the economical viewpoint of isoprene recovery. Due to the close boiling points of the 5 components, isoprene must often be concentrated by such expensive procedures as distillation or solvent extraction. Therefore, any improvement which can be made in isoprene concentration in the crude reaction product is very important. This is particularly true for the thermal cracking of 3,3-dimethyl-a-olefins and l-halo- 3,3-dimethyl alkanes of over 6 carbon atoms since these substances crack very selectively (greater than to C species but give only moderate concentrations of isoprene in the C fraction.

In addition to improving the yield of isoprene and the concentration of isoprene in the C fraction, conducting the cracking reaction in the presence of C C tertiary olefins also gives an increase in reaction rate so that at a given set of reaction conditions the conversion of the feed is greater. Furthermore, of great practical value is the fact that the C -C tertiary olefins formed in the cracking reaction need not be separated for recycle of the feed. This is important when the boiling point of the feed is very close to isoamylene as, for example, with 3,3-dimethyl-l-butene, B.P; 41 C. and Z-methylbutene-Z, HP. 38 C.

The olefin feedstock of this invention has the formula:

wherein R is an alkyl radical of 1 to 6 carbon atoms, straight or branch chained and with the higher number carbon atom radicals (i.e. of 3to 6 carbons), the branched structure being preferred. Suitable olefin feeds included, for instance, 3,3-dimethylbutene-l; 3,3-dimethylpentene- 1; 3,3-dimethylhexene-1; 3,3,4-11r-imethylpentene-1; 3,3- dimethyl-S,S-dimethylhexene-1; etc.

The 3,3-(lin1t3tl1Yl-ca-Ol6fin feed of the present invention can be prepared by any method known to the art. For instance, 3,3-dimethylbutene-1 may be obtained by a process which comprises dimerization of acetone by aluminum amalgum to form pinacol, rearranging the pinacol to pinacolone by treatment with an acid catalyst, reducing the pinacolone by catalytic methods to the corresponding alcohol, 3,3-dimethylbutanol-2, forming a Xanthate or other appropriate ester of the alcohol and subjecting it to thermopyrolysis to obtain 3,3-dimethylbutene-l. 3,3-dimethylbutene-1 has also been obtained by the pyrolysis of the stearic acid ester of 3,3-dirnethylbutanol-2 (see Koch and van Raay, Greenstoff-Chemie 32, 161-174, 1951). The preferred, and more practical process for the production of 3,3-dirnethyIbu-tene-1 is disclosed in application Serial No. 94,956 to Emmett H. Burk, Jr. and William D. Hoffman, filed January 11, 1961, now abandoned. Briefly the process of the application involves dehydrochlorinating 1-chloro-3,I3-dirnethylalkane by contact with a solid inert contact material at at temperature of about 450-600 C. and a residence time of about .01 to 10 seconds.

The 3,3-dimethyl alkyl halide feedstock of the present invention has the formula:

wherein R is an alkyl radical of 1 to 6 carbon atoms, straight or branch chained and with the higher number carbon atom radicals (i.e. of 3 to 6 carbons) the branched structure being preferred; and Z is a halogen such as chlorine, bromine, iodine and fluorine, preferably bromine. The feed materials of the present invention are readily available materials being produced by several methods well known in the art.

1-chloro-3,3-dimethylbutane, for example, can be prepared as described by Lewis Schmerling in the Journal of the American Chemical Society, 67, 115254 (1945). Briefly, the process involves reacting 1 mol of ethylene with t-butyl chloride using a Friedel-Crafts catalyst such as A101 FeCl BiCl or ZnCl The reaction can take place at atmospheric pressure when employing the reactive Friedel-Crafts catalyst such as AlCl advantageously at a temperature of about 15 C. Under the conditions listed by Schmerling, i.e. complete conversion, the yield of 1 chloro 3,3 dimethylbutane was reported as 75% theory. The other halides may be made by analogous procedures. Suitable alkyl halide feeds for use in the present invention, for instance, 1 halo 3,3- dimethylbutane; 1 ha-lo 3,3 dimethylpentane; l-halo- 3,3 dimethylhexane; 1 halo 3,3,4 trimethyl pent-ane, 1 halo 3,3 dimethyl 5,5 dimethylhexane; etc. The alkyl halides can be substituted with non-interferring groups if desired.

In accordance with the process of the present invention the feed is subjected to thermal cracking generally at a temperature of about 1000 F. to 1600 F. in the presence of at least one mol percent of a hydrogen halide, preferably 215 mol percent, based on feed and in the presence of at least about 3 up to about 75 mol percent of extraneous or added C C tertiary olefins, preferably about 5 to 30 mol percent based on the feed. The C -C tertiary olefins are present at the start of the reaction or at least substantially throughout the reaction period, that is, a reaction period which in the presence of the C -C tertiary olefins is sufficient to enhance the overall yield of isoprene. The tertiary olefins of the present invention include isobutylene and isoamylenes such as 2 methylbutene 1, 2 methylbutene 2 and 3- methylbutene- 1 The process can be conducted under vacuum or in the presence of an inert gas diluent such as helium, steam, etc. Operating conditions can be adjusted to give a feed partial pressure of up to about 0.5 atmosphere, for instance, .05 to 0.5 atmosphere, and a contact time sufficientto produce isoprene, for instance about .001 to 1.0 second. The inert gas, if employed, can be supplied in a molar ratio of at least about 1 mole, for instance, about 1 to 40 or 100 or more moles, preferably to 30 moles per mole of hydrocarbon. Under preferred conditions the partial pressure is about 0.1 atmosphere and the con tact time is about 0.05 to 0.1 second. A reaction temperature of about 1200 to 1400 F. is preferred. If desired, the inert gas can be employed as the beating medium to bring the feedstock rapidly to the cracking temperatures. This can conveniently be done by heating the inert gas to a temperature above that desired for conducting the cracking operation, generally at least about C. higher than the reaction temperature and not above say about 200 C. of the reaction temperature. The inert gas is then quickly mingled with the hydrocarbon Which is at a temperature below that at which any reactions occur. The temperature to which the inert gas is to be heated can be readily determined from the specific heat of the gas, the molar ratios involved, etc. The product from the thermal cracking process is quenched to a temperature of about 500 F. or below and fractionated in ordinary fractionating equipment to obtain isoprene of at least about 95% purity.

The hydrogen halide catalyst that may be used in the present invention can be hydrogen chloride, iodide or bromide per se or it can be the iodide or bromide compounds, which decompose and/or dissociate under the reaction conditions to produce hydrogen halide and ill 4 whose presence is not otherwise detrimental to the desired reaction. Illustrative of suitable halides are hydrocarbon halides such as methyl halide, tertiary butyl halide, carbon tetrachloride, or inorganic halides such as hydrogen halide. The preferred halide is the iodide. When the feed selected for pyrolysis in accordance with the method of the present invention is a l-bromoor 1-iodo-3,3- dimethyl alkane, the conditions of the cracking operation are such that sufficient HI or HBr is provided in the operation without the need for additional halide. However, if the halo carbon feed selected is the chloride, H1 or HBr should be provided to obtain the desired results.

The following examples will serve to illustrate the present invention.

EXAMPLE I A IZ-inch furnace was equipped with a Vycor reactor containing a fluidized bed of powdered quartz. 3,3-dimethyl-1 chlorobutane to which was added 7.3% by weight of isoamylenes was cracked in the presence of HI as catalyst under the conditions shown in Table I. For comparison two runs wherein isoamylenes were not added were also conducted. The results are summarized in Table I below:

Table I Run r 1, 211-55 1, 265-18 1, 265-78 Temperature, F 1, 800 1,325 1, 319 Contact time (sec) 0.25 1.00 .35 IIgO/IIC (molar). 21 11.7 10.6 111, mole pcrcen 4. 4 5. 2 4.7 Isoainylcncs in feed, wt. percent 0 0 7. 3

Products, ultimate Weight percent (chloride free):

42. 95 46. 38. 20 12. 95 9. 34 =8. 25 39. 45 36. 40 44. Others... 4. 65 7. 51 8. 65 Molar selectivity to isoprene, percent". 48. 6 45. 0 55.5

B This does not include the isoamylenes present in the feed which have been backed out to give a not yield of isoamylene.

Examination of the data in Table 1 demonstrates that inclusion of isoamylene in the 3,3-dimethyl-l-chlorobutane improved the molar selectivity to isoprene by 6.9% and 10% and offered an improved yield of isoprene relative to isoamylene.

Example 11 A 12-inch furnace was equipped with a Vycor reactor containing a fluidized bed of powdered quartz. 3,3-dimethyl-butene-l containing small amounts of isoamylene was introduced into a stream of steam and the mixture was passed through the reaction zone under the conditions shown in Table I below. The steam diluent was used at a mole ratio of 21/1 with hydrocarbon. 5 mole percent hydrogen iodide based on hydrocarbon was used as catalyst. The contact time of both runs Was 0.25 second. The results are summarized in Table II below.

We claim: 1. A process for producing isoprene which consists es sentially of subjecting to thermal pyrolysis a feed selected from the group consisting of (I) a 3,3-dirnethyl-a-olefin having the structural formula:

R- CH=GH;

CIII3 whergin R is an alkyl radical of l to 6 carbon atoms an (II) a halogenated hydrocarbon having the structural formula:

CH3 R'(|]'OH3CH2Z wherein R is an alkyl radical of 1 to 6 carbon atoms and Z is a halogen atom,

at a temperature of about 1000 F. to 1600 F. and a feed partial pressure of up to about 0.5 atmosphere and in the presence of at least about 1 mole percent of hydrogen iodide and at least about 3 mole percent of an extraneous isoamylene.

2. The process of claim 1 wherein the isoamylene is present in an amount of about 5 to 30 mole percent.

3. The process of claim 1 wherein an inert gas in a molar ratio of at least about 1 mole of inert gas per mole of feed is employed.

4. A process for producing isoprene which comprises subjecting 3,3-dimethyl butene-l to thermal pyrolysis at". a temperature of about l200 to 1400 F., a 3,3-dimethylbutene-l partial pressure of-up to about 0.5 atmosphere and in the presence of at least about 1 mole percent of HI and about 5 to mole percent of isoamylene.

References Cited by the Examiner FOREIGN PATENTS 598,259 4/61 Belgium.

605,237 10/61 Belgium.

868,5 66 5/61 Great Britain. 1,251,127 12/60 France. 

1. A PROCESS FOR PRODUCING ISOPRENE WHICH CONSISTS ESSENTIALLY OF SUBJECTING TO THERMAL PYROLYSIS A FEED SELECTED FROM THE GROUP CONSISTING OF (I) A 3,3-DIMETHYL-A-OLEFIN HAVING THE STRUCTURAL FORMULA: 